<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 500px;
            height: 500px;
            margin: auto;
            background-color: pink;

            display: flex;
            /* 水平居中 */
            justify-content: space-around;
            /* 垂直居中 */
            align-items: center;
        }

        .content {
            width: 70px;
            height: 70px;
            text-align: center;
            line-height: 70px;
            background-color: skyblue;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="content">内容</div>
    </div>
</body>

</html>